Personally, less than once per week. I find with the usual fluctuations in weight from day to day I get discouraged seeing mixed or limited results, I much prefer sticking to my calorie limit and exercise then jumping on the scales every so often, maybe once or twice a fortnight. I find this really helps with my own confidence by not only avoiding seeing the discrepancies each day or so, but by also seeing much bigger jumps in weight lost when I do check.